Riser assembly and method

ABSTRACT

A riser assembly and method of providing a riser assembly for transporting production fluids from a location deep under water are disclosed. The riser assembly includes a riser comprising a flexible pipe, the riser having a hang off point which is an end of the riser that in use connects with a floating facility, and a touchdown zone which in use is a section of the riser in the region of where the riser meets the seabed or other fixed structure; and at least one buoyancy element arranged at the touchdown zone or within about 50 m of the touchdown zone of the riser to support the riser in a configuration for accommodating kinetic energy propagating along the riser from the hang off point.

The present invention relates to a method and apparatus for providing a riser assembly. In particular, but not exclusively, the present invention relates to a riser assembly suitable for use in the oil and gas industry, especially in deep and ultra deep water applications, providing improved performance against problems associated with deeper water depth and excessive vessel motion.

Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 metres or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 metres. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows deflections to some degree, without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer or composite layers. Pipe body may alternatively be built up as a structure of only composite layers.

Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 metres)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.

A free hanging riser is a commonly used configuration for a flexible pipe riser for deep water application. Flexible pipe body as described above may be formed into a section of flexible pipe by the addition of one or two end fittings, respectively at the first and second ends of the pipe body. Sections of flexible pipe may then be joined end to end to form a riser. In this respect, a riser may be formed of one or several sections of flexible pipe.

However, in use, a free hanging riser may experience movement from vessel/platform motion on the sea surface. For example, movement at the top section of a riser induced by vessel/platform movement will likely propagate along (down) the riser. In a free hanging configuration, the riser generally lies in an almost vertical position, and as such the induced kinetic energy may easily propagate down the length of the riser. The energy will travel to the touch down zone of the riser, which is the region of the riser around the point that touches down and rests on the seabed (or other fixed platform) (touch down point). At this point, the seabed acts against the riser and excessive tension, compression and/or overbending of the flexible pipe may result. This may also lead to the formation of ‘small waves’ in the touch down zone, which may result in violation of the structural capacity of the flexible pipe, such as lateral buckling of wires known as birdcaging.

FIG. 1 illustrates the effect of vertical movements (indicated by the arrows) on a free hanging riser 100 that is suspended from a floating facility 102, in this case a ship, and extends down to lay on the seabed 104. The touch down zone 106 of the riser is highlighted in the box A. Box A is also shown in an expanded view as box A′. As can be seen in box A′, small waves 108 in the touch down zone of riser are present, caused by the vertical movement of the riser. These small waves are detrimental to the condition of the riser, and may lead to overbending, compression or crushing. This is particularly because the waves occur at the deepest part of the sea, having the greatest hydrostatic pressure on the flexible pipe. Therefore, the application of the free hanging riser may be limited by these effects.

A known way of trying to mitigating this problem is to change the whole configuration of the riser in the water by increasing the angle at the top hang off point (the section of the riser where the riser hangs from the vessel). FIG. 2 illustrates how the angle may be changed to try and help reduce the occurrence of small waves. The riser configuration 200 ₁ is similar to that shown in FIG. 1. The riser configuration 200 ₂ has the angle at the top hang off point 201 increased by placing the touch down zone further away (laterally) from the hand off point. However, this may not completely prevent the occurrence of the small waves and problems associated with vertical movement of a riser caused by vessel/platform movement in the water.

According to a first aspect of the present invention there is provided a riser assembly for transporting production fluids from a location deep under water, comprising:

-   -   a riser comprising a flexible pipe, the riser having a hang off         point which is an end of the riser that in use connects with a         floating facility, and a touchdown zone which in use is a         section of the riser in the region of where the riser meets the         seabed or other fixed structure; and     -   at least one buoyancy element arranged at the touchdown zone or         within about 50 m of the touchdown zone of the riser to support         the riser in a configuration for accommodating kinetic energy         propagating along the riser from the hang off point.

The phrase “at the touchdown zone or within about 50 m of the touchdown zone” thus encompasses the buoyancy element being located at any position at a point on the pipe where the riser may touch the seabed (during maximum and minimum extremes of dynamic, variable loading, lifting and shifting of the riser, i.e. the ‘touchdown zone’) to a point about 50 m away from the touchdown zone.

According to a second aspect of the present invention there is provided a method of providing a riser assembly for transporting production fluids from a location deep under water, comprising:

-   -   providing a riser comprising a flexible pipe, the riser having a         hang off point which is an end of the riser that in use connects         with a floating facility, and a touchdown zone which in use is a         section of the riser in the region of where the riser meets the         seabed or other fixed structure; and     -   providing at least one buoyancy element arranged at the         touchdown zone or within about 50 m of the touchdown zone of the         riser to support the riser in a configuration for accommodating         kinetic energy propagating along the riser from the hang off         point.

According to a third aspect of the present invention there is provided a riser assembly substantially as herein described with reference to the accompanying drawings.

According to a fourth aspect of the present invention there is provided a method substantially as herein described with reference to the accompanying drawings.

Certain embodiments of the invention provide the advantage that potential compression, tension, overbending or crushing of the riser brought on by excessive vessel motion are reduced or eliminated from at least greater part of the riser assembly configuration. Certain embodiments of the invention reduce or eliminate potential compression, tension, overbending or crushing from the entire riser assembly configuration.

Certain embodiments of the invention provide the advantage that small waves produced in the touchdown zone of a riser, and the associated problems, may be reduced or eliminated.

Certain embodiments of the invention provide the advantage that a riser assembly for deep and ultra deep water application is provided that can be installed relatively quickly and at relatively low cost.

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

FIG. 1 illustrates a known free hanging riser;

FIG. 2 illustrates further known free hanging risers;

FIG. 3 illustrates a flexible pipe body;

FIG. 4 illustrates a riser assembly;

FIG. 5 illustrates the riser assembly of FIG. 4 at a later time;

FIG. 6 illustrates a touchdown zone portion of another riser assembly; and

FIG. 7 illustrates a touchdown zone portion of yet another riser assembly.

In the drawings like reference numerals refer to like parts.

Throughout this description, reference will be made to a flexible pipe. It will be understood that a flexible pipe is an assembly of a portion of a pipe body and one or more end fittings in each of which a respective end of the pipe body is terminated. FIG. 3 illustrates how pipe body 300 is formed in accordance with an embodiment of the present invention from a combination of layered materials that form a pressure-containing conduit. Although a number of particular layers are illustrated in FIG. 3, it is to be understood that the present invention is broadly applicable to coaxial pipe body structures including two or more layers manufactured from a variety of possible materials. For example, the pipe body may be formed from metallic layers, composite layers, or a combination of different materials. It is to be further noted that the layer thicknesses are shown for illustrative purposes only.

As illustrated in FIG. 3, a pipe body includes an optional innermost carcass layer 301. The carcass provides an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of an internal pressure sheath 302 due to pipe decompression, external pressure, and tensile armour pressure and mechanical crushing loads. The carcass layer is often a metallic layer, formed from carbon steel, for example. The carcass layer could also be formed from composite, polymer, or other material, or a combination of materials. It will be appreciated that certain embodiments of the present invention are applicable to ‘smooth bore’ operations (i.e. without a carcass) as well as such ‘rough bore’ applications (with a carcass).

The internal pressure sheath 302 acts as a fluid retaining layer and comprises a polymer layer that ensures internal fluid integrity. It is to be understood that this layer may itself comprise a number of sub-layers. It will be appreciated that when the optional carcass layer is utilised the internal pressure sheath is often referred to by those skilled in the art as a barrier layer. In operation without such a carcass (so-called smooth bore operation) the internal pressure sheath may be referred to as a liner.

An optional pressure armour layer 303 is a structural layer with a lay angle close to 90° that increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal pressure sheath, and typically consists of an interlocked construction. The pressure armour layer is often a metallic layer, formed from carbon steel, for example. The pressure armour layer could also be formed from composite, polymer, or other material, or a combination of materials.

The flexible pipe body also includes an optional first tensile armour layer 305 and optional second tensile armour layer 306. Each tensile armour layer is a structural layer with a lay angle typically between 10° and 55°. Each layer is used to sustain tensile loads and internal pressure. The tensile armour layers are often counter-wound in pairs. The tensile armour layers are often metallic layers, formed from carbon steel, for example. The tensile armour layers could also be formed from composite, polymer, or other material, or a combination of materials.

The flexible pipe body shown also includes optional layers of tape 304 which help contain underlying layers and to some extent prevent abrasion between adjacent layers.

The flexible pipe body also typically includes optional layers of insulation 307 and an outer sheath 308, which comprises a polymer layer used to protect the pipe against penetration of seawater and other external environments, corrosion, abrasion and mechanical damage.

It will be appreciated that further layers may optionally be used. For example, further layers, e.g. metallic layers (i.e. layers of relatively heavy material), may be added for the purpose of adding weight to the flexible pipe. The flexible pipe may be a metallic pipe with a polymer inner and outer sheath. This may be useful in situations where it is needed to provide weight, or negative buoyancy, to a flexible pipe.

The typical weight of a flexible pipe according to the invention may vary depending on the structure, materials used and number of layers used. In certain flexible pipes, the pipe weight (kg/m) may vary at least between the values shown in Table 1, below (and could extend even further than these values). It can be seen that a flexible pipe may have a weight between around 25 to 500 kg/m. Considering pipes having a weight around the lower boundary (i.e. relatively “light” pipes), the weight may be between around 25 to 250 kg/m, depending on the inner diameter of the pipe. Considering pipes having a weight around the upper boundary (i.e. relatively “heavy” pipes), the weight may be between around 60 to 500 kg/m, depending on the inner diameter of the pipe.

The riser assemblies of the present invention may use any of these flexible pipes mentioned herein.

TABLE 1 Pipe inside Lower boundary Upper boundary diameter (inches) weight (kg/m) weight (kg/m) 2 25 60 4 40 110 5 60 150 6 90 180 9 170 300 10 190 340 16 250 500

Each flexible pipe comprises at least one portion, sometimes referred to as a segment or section of pipe body 300 together with an end fitting located at at least one end of the flexible pipe. An end fitting provides a mechanical device which forms the transition between the flexible pipe body and a connector. The different pipe layers as shown, for example, in FIG. 3 are terminated in the end fitting in such a way as to transfer the load between the flexible pipe and the connector.

The flexible pipe may be in segments of flexible pipe body with connecting end fittings.

An embodiment of the present invention is shown in FIG. 4. As is shown, a free hanging riser 400 is suspended from a ship 402 and extends down to lay on the seabed 404. The touch down zone 406 of the riser is highlighted in the box B. Box B is also shown in an expanded view as box B. The touchdown zone of a riser need not be at a specific linear position in the riser, but is the length of riser along which, at various times, it meets the seabed during maximum and minimum extremes of dynamic, variable loading, lifting and shifting of the riser.

As can be seen in box B, a set of buoyancy elements 410 ₁, 410 ₂ . . . 401 _(n) are affixed to the riser 410 in the touchdown zone 406. Here, ten buoyancy aids are affixed, though any number may be used according to the specific requirements of the situation. This will depend upon parameters of the riser, such as length, weight, and the conditions of use (sea movement, etc.), and can be determined by a person skilled in the art.

Buoyancy elements are known per se, and are added to a riser to provide an upwards lift to counteract the weight of a riser, effectively taking a portion of the weight of a riser. A buoyancy element may be a can filled with buoyant material such as air or syntactic foam for example, and can be clamped to a portion of flexible pipe. Usually, if required for deep water application, buoyancy elements would be provided at intermittent points along the length of the riser, such as disclosed in WO2007/125276. In other applications, buoyancy elements are not required for deep water use, and the suspended riser is known as a free hanging riser.

The buoyancy elements 410 ₁, 410 ₂ . . . 410 _(n) are provided in two groups, so as to change the shape of the riser sitting in the water in the touchdown zone. In this embodiment, the section of riser in the touchdown zone 406 has an almost horizontal profile from the direction viewed in the figure. The buoyancy elements 410 ₁, 410 ₂ . . . 410 _(n) are provided at a distance of up to around 50 metres from the seabed, aptly between 5 and 50 m, more aptly between 10 and 45 m, and more aptly between 20 and 40 m. FIG. 4 indicates the buoyancy elements provided within the touchdown zone of the riser in box B. In some embodiments the buoyancy elements may be arranged at the touchdown zone or within about 50 m of the touchdown zone of the riser (as indicated by the 50 m distance marker in FIG. 4).

By providing such a section, any movement caused by a vessel above, which transmits along the riser towards the touchdown zone, is dampened at the area of the buoyancy elements, since the vertical motion is transferred into bending motion between the two sections of buoyancy elements. The resulting movement of that section of riser is illustrated in FIG. 5, where it can be seen that the buoyancy sections form a curved section to absorb the movement. The motion will not therefore reach the point the riser touches the sea bed with the same level of energy. Essentially, the buoyancy elements cause a change in shape to the natural way in which a free hanging riser sits in the water, and this change in shape helps to dissipate kinetic energy travelling along that riser.

A further embodiment of the invention is shown in FIG. 6, which shows only the touchdown zone 606 of a riser assembly resting on seabed 604. The remaining portion of the riser will be as shown in FIGS. 4 and 5. As shown in FIG. 6, the touchdown zone of the riser 600 has buoyancy elements 610 ₁₋₃ arranged to form a ‘wave’ configuration, i.e. having a sag bend 612 and a hog bend 614.

By providing such a section, any upwards movement of the riser will be dampened by the sag bend 612 becoming shallower. Any downwards movement of the riser will be dampened by the sag bend 612 becoming deeper. Therefore any vertical movement caused by a vessel above will not therefore reach the point the riser touches the sea bed with the same level of energy.

A further embodiment of the invention is shown in FIG. 7, which shows only the touchdown zone 706 of a riser assembly resting on seabed 704. The remaining portion of the riser will be as shown in FIGS. 4 and 5. As shown in FIG. 7, the touchdown zone of the riser 700 has buoyancy elements 710 ₁₋₄ arranged to form a ‘double wave’ configuration, i.e. having sag bends 712,716 and hog bends 714,718. The double wave works in a similar manner to the embodiments described above, acting to dampen any kinetic energy caused by vessel movement.

In use, a riser will be payed out into the sea from a vessel or platform, and the required number of buoyancy elements will be attached to the riser at predetermined positions so as to form a riser assembly such as those illustrated in FIGS. 4 to 7.

Various modifications to the detailed designs as described above are possible. For example, the number and configuration of buoyancy aids can be determined to suit the specific requirements of use. One or more buoyancy element may be used. A plurality of buoyancy elements may be used. One or more subsections or groups of buoyancy elements may be used.

With the above-described arrangement, adverse effects on the touchdown zone of a flexible pipe caused by movement of a floating facility from which a riser is suspended may be reduced or eliminated. In particular, kinetic energy propagating along the riser from the hang off point may be accommodated.

By providing at least one buoyancy element arranged at the touchdown zone or within about 50 m of the touchdown zone of the riser, kinetic energy propagating along the riser from the hang off point may be accommodated, as the impacting weight of the riser hitting the seabed is controlled. As such, the likelihood of damage or breach to the riser, in particular to the outer sheath, is reduced. This is particularly important for a pipe made up of a mixture of metallic and either or both polymeric and or composite materials, as any breach in the outer layers of the pipe body may lead to corrosion damage and ultimately to failure of the pipe.

In addition, with the above-described arrangement, tension at the top hang off area of a riser can be reduced.

With the above-described arrangement, the advantages of a free hanging configuration are achieved, such as being cost effective in fabrication and in installation, and yet the disadvantages that are usually associated with a free hanging riser (small waves, overbending, compression, etc., as discussed above) are avoided.

With the above-described arrangement, the length of the touchdown zone of a riser will likely be reduced, since less or the riser should be repeatedly colliding against the seabed.

It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 

1. A riser assembly for transporting production fluids from a location deep under water, comprising: a riser comprising a flexible pipe, the riser having a hang off point which is an end of the riser that in use connects with a floating facility, and a touchdown zone which in use is a section of the riser in the region of where the riser meets the seabed or other fixed structure; and at least one buoyancy element arranged at the touchdown zone or within about 50 m of the touchdown zone of the riser to support the riser in a configuration for accommodating kinetic energy propagating along the riser from the hang off point.
 2. A riser assembly as claimed in claim 1, wherein the at least one buoyancy element is arranged on the riser at a distance of between around 5 and 50 m from the seabed or other fixed structure in use.
 3. A riser assembly as claimed in claim 2 wherein the at least one buoyancy element is arranged on the riser at a distance of between around 10 and 45 m from the seabed or other fixed structure in use.
 4. A riser assembly as claimed in claim 3 wherein the at least one buoyancy element is arranged on the riser at a distance of between around 20 and 40 m from the seabed or other fixed structure in use.
 5. A riser assembly as claimed in claim 1 wherein the at least one buoyancy element is arranged to form a sag bend in use.
 6. A riser assembly as claimed in claim 1 wherein the at least one buoyancy element is arranged to form a waved configuration in use.
 7. A riser assembly as claimed in claim 1 wherein the at least one buoyancy element is arranged to form a double waved configuration in use.
 8. A method of providing a riser assembly for transporting production fluids from a location deep under water, comprising: providing a riser comprising a flexible pipe, the riser having a hang off point which is an end of the riser that in use connects with a floating facility, and a touchdown zone which in use is a section of the riser in the region of where the riser meets the seabed or other fixed structure; and providing at least one buoyancy element arranged at the touchdown zone or within about 50 m of the touchdown zone of the riser to support the riser in a configuration for accommodating kinetic energy propagating along the riser from the hang off point.
 9. A method as claimed in claim 8 further comprising arranging the at least one buoyancy element on the riser at a distance of between around 5 and 50 m from the seabed or other fixed structure in use.
 10. A method as claimed in claim 9 further comprising arranging the at least one buoyancy element on the riser at a distance of between around 10 and 45 m from the seabed or other fixed structure in use.
 11. A method as claimed in claim 10 further comprising arranging the at least one buoyancy element on the riser at a distance of between around 20 and 40 m from the seabed or other fixed structure in use.
 12. A method as claimed in claim 8 further comprising arranging the at least one buoyancy element to form a sag bend in use.
 13. A method as claimed in claim 8 further comprising arranging the at least one buoyancy element to form a waved configuration in use.
 14. A method as claimed in claim 8 further comprising arranging the at least one buoyancy element to form a double waved configuration in use. 